With body on frame vehicles like Fenders and Disco's the I.D. stays with the chassis.
You could put a good body/interior from another car on your good chassis and it would retain the chassis VIN/reg' number. similarly you could fit a different engine, petrol, diesel, V8 or whatever and the car still retains its VIN/reg' number.
The upshot of all this is you don't need to do anything with your chassis VIN etc as long as all the bits are from the same type of car i.e. all the bits come from a Disco'1 or whatever. Fit the good bits from the donor and tell the DVLA about the engine swap, your insurance company won't care as long as you declare the correct engine.